Installations are known for producing electric power from wind power which comprise an alternator of which the rotor is driven by a rotating part of a wind machine having a hub and blades fixed on the hub.
In particular wind machines have been proposed in which the rotating part is connected directly to the rotor of the alternator without intermediate mechanical transmission. Such wind machines have the advantage of a greater mechanical simplicity, the rotating assembly comprising the rotating part of the wind machine integral with the rotor of the alternator being mounted so as to rotate on the structure of the wind machine via at least one block which may consist of one single bearing, in an advantageous embodiment.
Furthermore, the elimination of mechanical elements such as reducers with gears is reflected in a reduction in the costs of construction and of maintenance of the wind machine. Also the risks of breakdowns and deterioration of certain parts of the wind machine, for example by seizing of the gearing of a reducer, are avoided. However, the drawbacks of a direct connection between the rotating part of the wind machine and the rotor of the alternator are that the alternator must be able to operate in a satisfactory manner at a low speed of rotation and that the variable atmospheric conditions in which the wind machine functions, in particular the wind speed, can bring about variations in the electrical parameters of the current supplied by the wind machine to an electric network, these variations being generally unacceptable.
A particularly advantageous embodiment of the alternator of a wind machine used for the production of electric current uses an alternator rotor having permanent magnets generally disposed in a field rotating in the axial direction produced by the windings of a stator disposed facing the rotor. The alternator has a generally discoid shape, the magnets of the rotor and the windings of the stator being distributed circumferentially over surfaces in the form of discs.
French patent application FR-97 02808 filed by JEUMONT INDUSTRIE and FRAMATOME proposes the use of such a discoid alternator, the advantages of which have been indicated in the description of the patent application, for the production of electric power by such a wind machine.
So as to obtain an electric current which is stable and of good quality over the network supplied by the wind machine, it has also been proposed in the French patent application to associate with the alternator a power electronics module including a first ac-dc converter, such as a rectifier, and a second dc-ac converter, such as an inverter, which are intended to supply a stable alternating current of good quality to the network supplied by the wind machine.
The conversion of the alternating current produced by the alternator of the wind machine into direct current and the conversion of the direct current obtained into alternating current in effect permit freedom from the variations of operation of the wind machine due to the atmospheric variations and make it possible to supply the network with a current having a perfectly constant frequency (for example 50 Hz) with a very good control of the voltage and of the power factor of the supplied current.
However, in the French patent application there is no description of means permitting regulation of all the operating parameters of the wind machine, whether these parameters relate to the electrical operation of the alternator or are constituted by the speed of rotation of the rotary assembly of the wind machine.
A first problem which is posed within the framework of operation of the wind machines relates to the protection of the rotary assembly comprising the blades and the hub of the wind machine when the wind is very strong and reaches speeds which are likely to bring about racing of the wind machine and deterioration of the rotary assembly and/or the bearings of this rotary assembly.
The wind machines must be produced in such a way as to ensure disengagement of the rotating part when the wind speed exceeds a certain level; the performance of the blades of the rotary assembly then becomes very poor and racing is avoided.
In the disengagement zone the increase in the power of the wind is compensated for by a reduction in the performance of the blades; therefore the power is roughly constant.
An automatic disengagement is obtained for a certain wind speed due to the profiling of the blades of the rotary assembly and a speed of rotation which is imposed upon this rotary assembly. Such automatic disengagement systems may be designated as “stall” systems and generally comprise a rotary assembly in which the blades are mounted fixed on the hub.
Other systems, designated generally as “pitch” systems, use a rotary assembly in which the blades are mounted so as to turn on the hub about an axis perpendicular to the axis of rotation of the rotary assembly, generally by means of a bearing which ensures the rotary mounting of the foot of the blade, the device further comprising mechanical means which ensure the adjustment of the setting angle of the blade on the hub. These mechanical means are generally controlled in such a way that the setting angle of the blade is adjusted continuously during the operation of the wind machine. In the case of a wind of which the speed exceeds a predetermined speed limit, the system ensures the disengagement of the rotary assembly.
The “stall” system has the drawback of being definitely adjusted when the mounting of the blades on the hub of the rotary assembly has been ensured, as the disengagement is always effected for a predetermined wind speed.
In fact, the regulation of the wind machine is carried out in such a way that the rotary assembly turns at a nominal speed which is combined with the wind speed to cause disengagement in conditions which can only be changed by modification of the mounting of the blades on the hub in order to vary the setting angle by pivoting the blades about their longitudinal axis. This operation of changing the setting angle of the blades must be carried out manually and necessitates stopping of the wind machine during a period which may be relatively long and the intervention of staff responsible for this operation on the rotating part of the wind machine.
Independently of the problems due to the disengagement by strong wind, it is desirable to adapt the setting angle of the blades to the climatic conditions in such a way as to obtain the best possible recovery of the wind power. Such an adaptation must be carried out as a function of the climatic variations and, in particular, it is necessary to modify the setting of the blades in order to change from hot-weather operation (summer) to cold-weather operation (winter).
In fact the cold winter air is denser than in summer, when the air is warmer and lighter. Therefore the wind is more powerful during winter, such that it is desirable to modify the setting of the blades of the wind machines when the season changes. Such operations are ponderous and are reflected in operational losses both in terms of supply and sale of electric current. Provision may therefore be made not to modify the setting of the blades but, in this case, the wind machine must be dimensioned for operation in the winter period, which involves a loss of operation in the summer period.
It is common to provide two different modes of coupling of the electric generator permitting it to function at two different nominal speeds of rotation of the rotary assembly of the wind machine.
However, each of these two operating speeds is only adapted to one wind speed and, outside these ideal conditions, the power output of the wind machine is dissipated all the more as the conditions deviate from the ideal operating conditions.
Another drawback of the wind machines in which the blades have a fixed setting and which operate according to the fixed-speed “stall” system is that it is necessary to reach the nominal speed of rotation of the wind machine before connecting the generator to the network in order to avoid connection in conditions which are unacceptable for the network. It is therefore necessary to reach a sufficient level of power in order to start to exploit the wind machine for the supply of current.
The systems of the “pitch” type in which the blades are mounted with a continuously adjustable setting angle on the hub have appreciable advantages relative to the device operating according to the fixed-speed “stall” system. In particular, these systems permit operation at fixed speed or at variable speed and permit coupling to the network for low wind speeds. However, such systems are extremely fragile due to the mounting of the blades by means of a bearing and the use of a chain for regulation of the setting angle. A concentration of forces is produced on the blades, at the level of the bearing, and, due to the fact that the angles of rotation of the blades necessary for the setting are generally small, the bearing tracks undergo marking in the zone of contact with the bearing elements.
The setting angle regulating chain must include a means for control of the displacement of the blades which may be of the hydraulic, electric or electromechanical type. Such controls have a dynamic which is reflected in a relatively slow action of the regulating chain. Therefore the orientation of the blades is not always at the ideal value necessitated by the operating conditions. For this reason, in certain phases insufficient power is recovered or, on the contrary, excessive power is recovered, which brings with it drawbacks in the operation of the mechanical or electrical transmission of power.
The rotary assemblies of the wind machines must, on the other hand, be produced in such a way that complementary braking of the wind machine can be ensured, for example by normal stopping or accidental stopping, in combination with a mechanical braking system disposed on a shaft of the rotating part. Therefore the rotating part must include a braking device, generally of the aerodynamic type.
In the case of a fixed-speed “stall” system, a braking element is provided on the free end of the blades which is oriented in such a way as to ensure aerodynamic braking.
In the case of a “pitch” system, the braking can be effected by feathering of the blades by means of the setting angle regulating chain.
In one or the other case, when a micro-cutoff appears on the network, that is to say a cutoff of the circulation of the electric current lasting typically less than a second, stopping of the wind machine must be commanded, by braking, in order to avoid any risk of deterioration of the electric generator. Following the stoppage, it is necessary to provide a procedure for restarting so that this mode of operation with stopping of the wind machine is reflected in a loss of production and fatigue of the mechanical elements of the wind machine and in particular of the mechanical brake used for stopping and of the blades.
The electrical networks which ensure the distribution of current to users require the voltage and the frequency of the current supplied to be as constant as possible; in the case of a network supplied by a wind machine the said voltage and frequency depend upon the speed of rotation of the generator and thus of the wind machine.
In the case where it is necessary to provide a wind machine for a network using a frequency different from the usual frequency (for example 60 Hz instead of 50 Hz), it is necessary to modify the generator (and possibly a multiplier) in order to adapt them to the required frequency.
In the case of a network in which the frequency is not very stable and varies relative to the predetermined nominal frequency, it is necessary to adapt the operation of the wind machine.
If the frequency required on the network is higher than the predetermined frequency, the speed of rotation of the wind machine must increase and consequently the power supplied increases.
If the frequency required on the network decreases, the speed of rotation of the wind machine decreases and therefore the power supplied decreases.
The power of the wind machine is not perfectly controlled, and for this reason it is necessary to provide oversizing of the wind machine in order to meet all the demands, which involves additional costs.
Finally, the rotary assembly of the wind machine does not turn at a perfectly constant speed and undergoes periodic variations due to the passage of the blades of the rotary assembly in front of the mast supporting the nacelle on which the rotary assembly is mounted. Therefore the current generator does not turn at a perfectly regulated speed ensuring perfectly stable and constant operation.